Triarthrus

Triarthrus
Temporal range: Upper Ordovician
Triarthrus eatoni, 11mm
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Trilobita
Order: Ptychopariida
Suborder: Olenina
Family: Olenidae
Subfamily: Triarthrinae
Genus: Triarthrus
Green, 1832
Species
  • T. beckii Green, 1832 (type species) = Paradoxides beckii, Calymene beckii
  • T. billingsi Barrande, 1872
  • T. canadensis Smith, 1861
  • T. eatoni (Hall, 1838) = T. macastyensis
  • T. glaber Billings, 1859
  • T. huguesensis Förste, 1924
  • T. latissimus Månsson, 1998
  • T. linnarssoni Torslund, 1940
  • T. rougensis Parks, 1921
  • T. sichuansis Lu & Chang, 1974
  • T. spinosus Billings, 1857

Triarthrus is a genus of Upper Ordovician ptychopariid trilobite found in New York, Ohio, Kentucky, and Indiana, eastern and northern Canada, China and Scandinavia. It is the last of the Olenid trilobites, a group which flourished in the Cambrian period. The specimens of T. eatoni that are found in the Beecher's Trilobite Bed, Rome, New York area are exquisitely preserved showing soft body parts in iron pyrite. Pyrite preservation has given scientists a rare opportunity to examine the gills, walking legs, antennae and digestive systems of trilobites, which are rarely preserved. Triarthrus is therefore commonly used in science texts to illustrate trilobite anatomy and physiology.

Distribution

Taxonomy

Cladogram showing the assumed relationships and known temporal distribution of the genera Bienvillia, Porterfieldia and Triarthrus, family Olenidae[2]

T. beckii and T. eatoni have long been considered closely related and possibly synonymous. Recent comparative analysis showed that there is no sharp distinction between the two, but that they seem to represent opposing morphotypes. T. beckii dominates earlier in the distribution and in shallow water, while T. eatoni does so later and in deeper waters. It is probable that the transgression of the sea level aided the eventual disappearance of the T. beckii morph. The occurrence of the supposed pluriform species continued for more than two million years.[3]

Species previously assigned to Triarthrus

Description

Exoskeleton

Reconstruction of Triarthrus eatoni, showing the appendages
Pyrite preserved Triarthrus eatoni - ventral side

Triarthrus is an average size trilobite (up to about 5 centimetres or 2.0 inches) and its moderately convex body is about twice as long as wide (excluding spines). Like in all Olenidae, the headshield (or cephalon) of Triarthrus has opisthoparian sutures, and the right and left free cheeks that they define are yoked. The cephalon in Triarthrus is semicircular. The central raised area (or glabella) is approximately quadrate, and considerably wider than the posterior margin of the fixed cheeks (or fixigenae). The front of the glabella is close to the anterior border (or the preglabellar field is short), but the border furrow is absent in front of the glabella. The facial suture crosses in front of the glabella on the top of an inflated rim. The posterior end of the glabella consists of the occipital ring that is defined by a furrow crossing over the midline. The occipital ring carries a small node at its centre point, and may be adorned with a backward directed spine as long as the glabella. In front of the occipital furrow two pairs of lateral furrows emerge from the axial furrow almost perpendicularly and curve backwards as they cut toward the midline, but without reaching it. Anteriorly, one or two further furrows are isolated shallow slits or depressions. The fixigenae are very narrow (less than ¼ of the width of the glabella at the back of the eye, and less than ½ at the occipital ring). The librigenae are narrow, and with or without spines. The border is prominent elsewhere, and the eyes of small to medium size, sitting at the end of small palpebral lobes next to the frontal half of the glabella. The thorax consists of 13 to 16 segments, with the axis wider than pleural regions, obliquely truncated or rounded pleural tips, and the fulcrum placed very close to the axis. The tailpiece (or pygidium) is small, with 3 to 5 axial rings, distinctly segmented pleural fields, and an entire, evenly rounded margin.[2][4]

Uncalcified parts

The foremost pair of extremities in trilobites are the antennas. In Triarthrus eatoni the antennas are probably flexible. From their attachment adjacent to the hypostome they stretch inward and forward, almost touching each other at the point where, from a dorsal viewpoint, they appear from under the cephalic margin, then bending outward and then forward again in a slight S-curve. They are about twice as long as the headshield (or cephalon), each consisting of 40 to 50 segments that are shorter than wide. As the preservation of soft tissue in T. eatoni is excellent, it is probable that it lacked cerci, the most backward pair of extremities known from Olenoides serratus.[4][5][6][7]

Key to the species

Triarthrus spinosus, 14mm excluding spines

For the five most common North-American species.[2]

1 Facial sutures cross the border furrow in front of the glabella. → 2
- Border furrow absent in front of the glabella. → 3
2 Glabella separated from the border by a preglabellar field. → Bienvillia
- Glabella contacts the frontal rim. → Porterfieldia
3 Cephalon without genal spines. → 4
- Cephalon with genal spines. → 5
4 Midpoint of the eye opposite the most anterior furrow that incises the glabella from the lateral furrow. → Triarthrus eatoni
- Back of the eye opposite the most anterior furrow. → Triarthrus becki
5 Glabella rounded anteriorly, genal spines directed approximately backward and slightly curving inward. → 6
- Glabella rounded anteriorly, genal spines straight, directed backward and 30°-45° outward. → Triarthrus canadensis
6 The occipital ring is adorned with a node midlength at the midline only. 9th thorax segment with a long backward directed spine. → Triarthrus rougensis
- The occipital ring carries a long backward directed spine at its backrim in addition to a node at midlength. 8th, 9th and 10th thorax segments carry long backward directed spines. → Triarthrus spinosus

Life cycle

Due to the existence of an excellently preserved deposit in the Frankfurt Shale near Rome, New York State, that suggests an entire population of Triarthrus eatoni was killed and quickly buried, it has been possible to make a plausible reconstruction of its life history. After hatching, probably from eggs a few tenths of a millimeter (although these are not known in any trilobite), the small protaspid and early meraspid stages supposedly lived between the plankton in the water column. This is derived from the fact that only exuviae of the early stages were found, making clear that they haven't died in the disaster that killed their elders. In this phase the larvae probably got dispersed over large areas. After a few months, at about 2mm long T. eatoni started living at the sea floor. Here, it must have fed by filtering suspended food particles, probably comparable to the trunk limb filter-feeding Cephalocarida and Branchiopoda. The exoskeletons fall into five size categories, suggesting that T. eatoni had a distinct breeding season, likely annually, as is the case in almost all extant crustaceans (although some amphipods have two breeding periods per year). It could live through at least four breeding seasons, and at that age reach approximately 4 cm in length.[8]

References

  1. 1 2 3 Liberty, B.A. (1969). "Palaeozoic geology of the Lake Simcoe Area, Ontario". Geological Survey of Canada Memoirs (355): 1–201. cited in Paleobiology Database. "Triarthrus canadensis". Retrieved 13 November 2013.
  2. 1 2 3 4 Ludvigsen, R.; Tuffnell, P.A. (1983). "A revision of the Ordovician olenid trilobite Triarthrus". Geological Magazine. 120 (6): 567–577. doi:10.1017/s0016756800027722.
  3. ↑ Cisne, John L.; Molenock, Joane; Rabe, Bruce D. (1980). "Evolution in a cline: the trilobite Triarthrus along an Ordovician depth gradient". Lethaia. 13 (1): 47–59. doi:10.1111/j.1502-3931.1980.tb01029.x.
  4. 1 2 Moore, R.C. (1959). Arthropoda I - Arthropoda General Features, Proarthropoda, Euarthropoda General Features, Trilobitomorpha. Treatise on Invertebrate Paleontology. Part O. Boulder, Colorado/Lawrence, Kansas: Geological Society of America/University of Kansas Press. pp. 1–560. ISBN 0-8137-3015-5.
  5. ↑ Beecher, C.E. (1893). "A larval form of Triarthrus". American Journal of Science. 46: 361–362.
  6. ↑ Beecher, C.E. (1893). "On the thoracic legs of Triarthrus". American Journal of Science. 46: 467–470.
  7. ↑ Beecher, C.E. (1902). "The ventral integument of trilobites". American Journal of Science, series 4. 13: 165–173.
  8. ↑ Cisne, John L. (1973). "Life History of an Ordovican Trilobite Triarthrus eatoni". Ecology. 54 (1): 135–142. doi:10.2307/1934382.

Sources

Further reading

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